The present invention relates to the field of the shaping of digital signals with a view to their transmission, and especially to a method and a device for forming transport frames from coded-signal frames emanating from a source.
The invention applies in particular, but not exclusively, to the transmission on a radio channel of voice signals from a vocoder.
The channel coding procedures used to form transport frames implement error detection and/or correction techniques using redundant codes applied to the coded-signal frames. Often, the bits of the frames produced by the source are cataloged into several classes to which are applied protection measures which are more or less effective against transmission errors, to achieve a compromise between the need to detect or correct any errors as a function of the importance of the information transmitted and the bandwidth required thereby.
In the example of voice communications in the GSM cellular radiocommunication system, the vocoder produces 260 bits per coded-signal frame of 20 ms, of which 50 bits are in class C1a, 132 bits are in class C1b and 78 bits are in class C2. The mechanism for forming the transport frames from the frames of the vocoder is a synchronized mechanism, one transport frame being produced every 20 ms for each voice frame. The bits of classes C1a and C1b are protected by a convolutional code of rate ½ allowing the receiver to correct transmission errors. Before this coding, 3 parity bits are appended to the 50 bits of class C1a to allow the detection of residual errors among these bits, which are the most sensitive. The transport frame formed by the 378 bits produced by the convolutional coder and by the 78 bits of class C2 is transmitted on the radio channel with temporal interleaving with other transport frames. Interleaving is envisaged to best utilize the correction capabilities of the convolutional code given the type of propagation channel of the GSM system.
In the GSM system, it is also envisaged that the physical channel on which the transport frames are transmitted in respect of a given communication can be shared with a fast signaling logical channel associated with this communication. This signaling channel, called the FACCH (“Fast Associated Control Channel”), is formed by a frame stealing mechanism: it gives rise to the loss of a transport frame corresponding to an output frame from the vocoder. This results in a loss of quality at the receiver level, which is only permissible because the stolen frames are in principle rare.
The frame stealing mechanism is not suitable when, on the same physical channel, a non-negligible bandwidth relative to that of the voice communication is needed. This occurs for example when signaling information representing a significant bit rate is multiplexed on the same physical channel as the speech signal. Another case is that of a frequency division multiple access (FDMA) cellular mobile radio-communication station which, when it is communicating with a base station on a traffic channel, has time intervals for regularly monitoring beacon channels formed by neighboring base stations so as, as the case may be, to allow the communication to be handed over to the base station affording the best radio propagation conditions (see for example French Patent Application No. 99 06345).
Typically, for this kind of application, it is desirable to deploy a channel coding scheme making it possible to transmit the N coded-signal frames emanating from a vocoder for a duration T in M transport frames available for this duration T, to free part of the transmission resource for this duration T to make it possible to multiplex other logical channels or to reserve windows for other functions, e.g. monitoring.
An object of the present invention is to propose such a scheme.